Make __all__ expose classes for use in type annotations

[OscarVanL]

Is your feature request related to a problem? Please describe.
I would like to type annotate my Pubsub subscriber callback as:
callable[[pubsub_v1.subscriber.message.Message], None] to match the callback function signature described here: https://cloud.google.com/pubsub/docs/publish-receive-messages-client-library

However, when I do this, my IDE (PyCharm) complains saying: Cannot find reference 'message' in '__init__.py'

This is because pubsub_v1.subscriber.__init__.py only has __all__ = ("Client",), meaning this is all that is suggested as an exported value.

Describe the solution you'd like
Expose the pubsub_v1.subscriber.message.Message in __all__ so I can add type annotations. I expect there are other types that it would be helpful to expose here too.